In the case of the ordinary manufacturing and processing operation of semiconductor wafers, a silicon single crystal ingot is first cut to slices of a given thickness by a wire saw, ID saw or the like to produce wafer substrates. The surface of these wafer substrates have minutes irregularities caused by the slicing or these wafer substrates are not uniform in thickness so that they are lapped to flatten the surface irregularities and to make the work distortions uniform in depth thereby preparing them to have the uniform thickness.
After the lapping, a work strained layer is caused by these processing steps and particles such as minute metals and abrasive powder, silicon wastes, etc., are deposited on the work strained layer thus requiring to perform an etching by a chemical process using strong acid, fluoric acid or the like to remove these particles.
Following the etching process, the wafer substrates are subjected to a single side mirror polishing after the acids deposited on their surfaces have been neutralized by alkali, rinsed in clean water and dried. Usually, the mirror polishing includes two levels of polishing composed of a rough polishing and a final polishing and finally they are advanced to a final cleaning process after the improvement of the minute surface roughness or microroughness as well as the removal of haze have been effected.
The semiconductor wafer substrates thus having the highly cleaned surfaces as the result of the final cleaning are subjected to an epitaxial growth process for depositing and growing silicon single crystals on the surface of the substrates by, for example, a H-Si-Cl system CVD process.
However, there has been a problem that even if a highly clean surface is obtained by a final cleaning, there is the danger of the cleaned surface being contaminated or damaged to cause defects until the loading in the furnace in the epitaxial growth process. For example, if the silicon surface is directly contaminated with metal or organic matter after the clean surface has been produced by effecting a fluoric acid treatment (HF treatment) in the final cleaning step, there is the danger of causing the formation of a layer of chemical compound due to such contaminants. This chemical compound layer has a strong bonding strength and it is not easily removable. Also, it has been confirmed that the wafer surface subjected to the HF treatment becomes extremely active thus tending to cause redeposition of particles of organic material or the like.
As a result, methods which protect the clean surface of a wafer subjected to the final cleaning until the following process to prevent such contamination have recently been studied. For instance, Japanese Patent No. 2,540,690 published on Oct. 9, 1996 discloses that in order to protect the wafer surface from damage in the form of defects, contamination, etc., an oxide film having a film thickness of 50 to 200 angstrom (5 to 20 nm) is intentionally formed on the surface of a semiconductor wafer before its loading in an epitaxial growth furnace and the formation of an epitaxial growth layer is performed after the oxide film has been removed in an ambience of HCI gas at 1,200.degree. C. in the furnace after the loading.
However, this conventional method gives rise to the following problem due to the removal of the oxide film by the high temperature HCI gas etching. More specifically, due to the fact that the reaction rate of HCI with Si is higher than its reaction rate with SiO.sub.2, even if a very small film thickness variation exists partly in the oxide film, as the HCI etching proceeds, minutes irregularities are caused resulting in surface roughness and this surface roughness causes crystal defects in the following epitaxial growth process.
On the other hand, while it has been known that such surface roughness due to HCI becomes increasingly eminent with decrease in the etching temperature, the recent tendency is toward performing the epitaxial growth at a relatively low temperature of about 1,000.degree. C. by using, for example, monosilane (SiH.sub.4) for the silicon source gas and therefore any attempt to avoid the labor of heating the interior of the epitaxial growth furnace to a higher temperature has the converse effect of making the problem of surface roughness unavoidable. Further, in the case of epitaxial growth at such low temperatures, such problems as the back contamination from HCI and the contamination due to any equipment corrosion by HCI cannot also be disregarded.
It will be seen from the forgoing that there has heretofore been established no method capable of satisfactorily producing a highly clean wafer surface without causing the occurrence of surface roughness and any new and additional contamination after the removal of an oxidation preventive film.
In view of the foregoing deficiencies, it is an object of the present invention to provide a method for manufacturing a semiconductor wafer capable of removing a protective oxide film in such a manner that the occurrence of surface roughness and contamination is reduced than previously thereby producing a highly clean wafer surface.